Atlas Copco XAS 175 Dd, Instruction Manual

Page: 13 / 34

2.5 A
IR FLOW
(
SEE
F
IG
. 2.3)
The system comprises:
AF Air filter
AR/OS Air receiver/oil separator
CE Compressor element
UV Unloader valve
BDV Blow-down valve
Air drawn through the airfilter (AF) into the compressor
element is compressed. At the element outlet, compressed air
and oil pass into the air receiver/oil separator (AR/OS).
The check valve prevents blow-back of compressed air when
the compressor is stopped. In the air receiver/oil separator
(AR/OS), most of the oil is removed from the air/oil mixture; the
remaining oil is removed by the separator element.
The oil collects in the receiver and on the bottom of the
separator element.
The air leaves the receiver via the minimum pressure valve
(MV), which prevents the receiver pressure from dropping
below the minimum working pressure (see section 8), even
when the air outlet valves are open. This ensures adequate oil
injection and prevents oil consumption. On units with an
aftercooler, the compressed air is discharged through the
outlet valve(s) via the aftercooler, where the air is cooled down
to approx. 25 °C above the ambient temperature and water
separator. When the unit is stopped, the non-return valve of
the minimum pressure valve (MV) prevents blow-back and the
blow-down valve (BDV) depressurises the air receiver (AR).
A temperature gauge (TG) and a working pressure gauge
(PG) are comprised in the system.
2.6 O
IL SYSTEM
(
SEE
F
IG
. 2.3)
The system comprises:
AR/OS Air receiver/oil separator
OC Oil cooler
OF Oil filter
The lower part of the air receiver (AR) serves as oil tank.
Air pressure forces the oil from the air receiver/oil separator
(AR/OS) through the oil cooler (OC) and oil filter (OF) to the
compressor element (CE).
The compressor element has an oil gallery in the bottom of its
casing. The oil for rotor lubrication, cooling and sealing is
injected through holes in the gallery.
Lubrication of the bearings is ensured by oil injected into the
bearing housings.
The injected oil, mixed with the compressed air, leaves the
compressor element and re-enters the air receiver, where it is
separated from the air as described in section 2.5. The oil that
collects in the bottom of the oil separator element is returned
to the system through scavenging line (SL), which is provided
with a flow restrictor (FR).
The oil filter by-pass valve opens when the pressure drop over
the filter is above normal because of a clogged filter. The oil
then by-passes the filter without being filtered. For this reason,
the oil filter must be replaced at regular intervals (see section
4.2).
2.7 S
PEED REGULATING AND UNLOADING SYSTEM
(
SEE
F
IG
. 2.3)
The system comprises:
RV Regulating valve
UV Unloader valve
SR Speed regulator
When the air consumption increases, the air receiver pressure
will decrease and vice versa. This receiver pressure variation
is sensed by the regulating valve which, by means of control
air to the unloader valve (UV), matches the air output to the air
consumption. The air receiver pressure is maintained between
the pre-selected working pressure and the corresponding
unloading pressure.
The air output is controlled from maximum output (100%) to no
output (0%) by:
1. Speed control of the engine between maximum load speed
and unloading speed (the output of a screw compressor is
proportional to the rotating speed).
2. Air inlet throttling.
If the air consumption is equal to or exceeds the maximum air
output, the engine speed is held at maximum load speed and
the throttle valve (TV) is fully open.
If the air consumption is less than the maximum air output, the
regulating valve (RV) supplies control air to speed regulator
(SR) and unloader valve (UV) to reduce the air output and
holds air receiver pressure between the normal working
pressure and the corresponding unloading pressure of
approx. 1.5 bar above the normal working pressure.
When the air consumption is resumed, the throttle valve (TV)
gradually opens the air intake and the speed regulator (SR)
increases the engine speed.
The construction of the regulating valve (RV) is such that any
increase (decrease) of the air receiver pressure above the
pre-set valve opening pressure results in a proportional
increase (decrease) of the control pressure to the throttle
valve (TV).
Part of the control air is vented to atmosphere, and any
condensate discharged, through the venting valve (VV).
I
NSTRUCTION
M
ANUAL
13